Carbon foams were first developed by researchers in the late 1960's. Shortly thereafter, processes were developed for controlling the structure and material properties of the carbon and the graphitic foams. A variety of applications for these materials were developed in the following decades and numerous different pre-cursors were developed for producing carbon foams.
For example, in the 1970's, carbon foam was produced from cork. In 1997, James W. Klett, at the Oak Ridge National Laboratory, reported the first graphitic foams with bulk thermal conductivities greater than 40 Watts per meter-Kelvin (W/m*K) and recently conductivities up to 180 W/m*K have been measured. This thermal conductivity may be compared to 400 W/m*K for copper. At a density of 0.45-0.6 g/cm3 for the graphite foam compared to a density of 8.9 g/cm3, the graphite foam has a specific thermal conductivity (thermal conductivity divided by density) of more than 4 times that of the copper. I.e. a specific thermal conductivity of 300 for the graphite foam to 45 for copper.
One method for producing carbon foam is described in U.S. Pat. No. 6,656,443 filed by James W. Klett on Jul. 26, 2002 entitled “Pitch-based Carbon Foam and Composites.” In this patent, carbon foam is manufactured from pitch. For example, Mitsubishi ARA24 mesophase pitch was utilized.
Other methods and variations of making carbon based foam articles are described in the following U.S. Pat. No. 7,258,836 “Freeze resistant buoy system;” U.S. Pat. No. 7,166,237 “Pitch-based carbon foam heat sink with phase change material;” U.S. Pat. No. 7,157,019 “Pitch-based carbon foam heat sink with phase change material;” U.S. Pat. No. 7,147,214 “Humidifier for fuel cell using high conductivity carbon foam;” U.S. Pat. No. 7,070,755 “Pitch-based carbon foam and composites and use thereof;” U.S. Pat. No. 7,014,151 “Pitch-based carbon foam heat sink with phase change material;” U.S. Pat. No. 6,809,304 “High efficiency, oxidation resistant radio frequency susceptor;” U.S. Pat. No. 6,780,505 “Pitch-based carbon foam heat sink with phase change material;” U.S. Pat. No. 6,673,328 “Pitch-based carbon foam and composites and uses thereof;” U.S. Pat. No. 6,663,842 “Pitch-based carbon foam and composites;” U.S. Pat. No. 6,656,443 “Pitch-based carbon foam and composites;” U.S. Pat. No. 6,430,935 “Personal cooling air filtering device;” U.S. Pat. No. 6,399,149 “Pitch-based carbon foam heat sink with phase change material;” U.S. Pat. No. 6,398,994 “Method of casting pitch based foam;” U.S. Pat. No. 6,387,343 “Pitch-based carbon foam and composites;” U.S. Pat. No. 6,344,159 “Method for extruding pitch based foam;” U.S. Pat. No. 6,287,375 “Pitch based foam with particulate;” U.S. Pat. No. 6,261,485 “Pitch-based carbon foam and composites;” U.S. Pat. No. 6,037,032 “Pitch-based carbon foam heat sink with phase change material;” and U.S. Pat. No. 6,033,506 “Process for making carbon foam.”
In one embodiment of the present invention, the carbon foam may be produced in the manner described above using pitch powder, granules, or pellets. However, the carbonizing heat treatments at the end of the process are replaced with a specialized form of heat treatment. In one embodiment, a carbon foam article that is substantially not graphitized is placed in an environment of an inert gas such as argon, and one surface of the article is exposed to radiant heat, preferably pulsed high density infrared heat, that is sufficient to raise the surface temperature of the article to a temperature above a graphitizing temperature (the temperature at which the foam changes from carbon to graphite). In this example, the graphitizing temperature for this particular precursor is about 2,000 degrees centigrade. So, the surface temperature of the article is heated to a temperature above 2,000 degrees centigrade. At this point, the foam begins to graphitize (become graphite) and becomes highly thermally conductive. Thus, the heat from the pulsed high density infrared source is quickly transferred through the graphitized portion of the foam to the interior portion of the foam core and it too is heated to a temperature above the graphitizing temperature.
As the heating process continues, the graphitizing temperature penetrates deeper and deeper into the core of the carbon foam, graphitizing the foam as the temperature within the core reaches the graphitizing temperature. By carefully and evenly applying the pulsed high density infrared heat to the surface of the graphitic foam, the penetration of the graphitizing temperature can be controlled to a desired depth. The power, duration, and duty cycle of heat pulses required to achieve the graphitizing temperature to a particular depth of an article will vary from article to article depending upon the size and properties of the article.
Radiant pulses of heat may be used to help achieve a sharp divide between the graphite portion and the carbon portion of the foam article. When a pulse is applied, the graphite is highly conductive thermally, and it quickly conducts the heat (thermal energy) to the interior edge of the graphite portion, where it heats the carbon portion of the foam. Since carbon foam is a poor thermal conductor, the temperature at the interface between the graphite portion and the carbon portion may be raised quickly to the graphitizing temperature, and a layer of the carbon foam is thereby graphitized. The temperature gradient is initially very steep at the interface between the graphite and the carbon foam. Then the heat pulse ceases and the graphite quickly cools, which quickly cools the interface region as well. By repeatedly heating and cooling the graphite portion of the foam block, the temperature gradient at the carbon-graphite interface remains very steep with each pulse, and thus the layer of carbon that is graphitized is sharply defined with each pulse.
At the depth to which the graphitizing temperature penetrates, there is an interface where the carbon foam is partially graphitized. The thickness (depth) of this interface may be controlled by the power and duration of the applied thermal pulses. To achieve a relatively thinner interface depth, the pulses may be applied with a relatively higher power and for a relatively shorter duration. To achieve a thicker interface depth, the pulses are applied with a relatively lower power and for a relatively longer duration. The more gradual heating provided by the lower power pulses tends to cause a more gradual heat gradient in the foam, which in turn produces a more gradual and larger interface of partially graphitized carbon foam.
One embodiment of the article is a porous carbon based foam article having a first region of graphitic carbon foam material, which may be pitch-based, having a relatively high thermal conductivity; and a second region of porous non-graphitic carbon foam, which may be pitch-based, contiguous with the first region and further comprising essentially non-graphitic carbon foam having a relatively low thermal conductivity, wherein the thermal conductivity of the graphitic foam is substantially larger than the thermal conductivity of the non-graphitic carbon foam.